1. Field of Invention
The present disclosure relates to topology architecture of a cascade inverter. More particularly, the present disclosure relates to a control device and a control method of the topology architecture during bypassing of power units.
2. Description of Related Art
Currently, a traditional middle, high voltage inverter is subject to the voltage rating of power devices, and moreover, in order to reduce harmonic level of the output voltage, a topology structure of a cascade inverter is usually adopted. In the topology structure, each phase is composed of several series connected power units with a lower voltage rating and could output a high voltage. However, due to the large number of the power units, the reliability of the system might be low since one or more power units have faults. At the moment, the fault power unit should be bypassed and the remain power units will be controlled to output the three-phase voltage in order to ensure continuous operation of the whole cascade inverter system. Meanwhile, after the fault power unit is bypassed, the original control method should be changed, otherwise the three-phase output voltage will be unbalanced.
In the prior art, there are three popular solutions. The first one is same-stage bypass. When faults happen to several power units of one phase, the corresponding power units at the same stage of the other two phases are controlled to be bypassed even without fault, so that a same number of the power units in each phase are assured to work. However, the control method sacrifices the power units without faults and significantly decreases the output voltage. The second one is decreasing the command voltage. Specifically, the command voltage of the phase having fewer fault power units is decreased to guarantee each phase output voltage equal. However, by using this method, not only is the output voltage decreased significantly, but also each of the power units has a great difference in output power and has uneven stress. The third one is neutral point voltage shift. That is, when faults happen to one or more power units of one phase, by changing a phase difference among all of phases, a line to line voltage of the output voltage still can be maintained balanced even when the phase voltage is unbalanced (because the voltage applicable to the load is the line to line voltage). However, the control method requires that all combinations of power units which are possibly bypassed in each of the phases should be exhausted, and the angle between each two phases after bypassing should be calculated offline for each of the combinations. Therefore, a large storage space of the controller is occupied. Additionally, since each phase should be controlled separately, the method is not suitable for a Space Vector Pulse Width Modulation (SVPWM) method. Therefore, the application thereof is restricted in terms of vector control of a converter.
In view of the above, many in the industry are endeavoring to find ways to design a control method of a cascade inverter during bypassing of power units to effectively solve or eliminate defects or disadvantages of the control method above.